The present invention relates to a potentiometric titration method and a potentiometric titration apparatus, and more particularly, to a potentiometric titration method capable of accurately determining a terminal point of titration irrespective of occurrence of change in indicator potential, and a potentiometric titration apparatus for performing automatic titration according to the potentiometric titration method.
In the potentiometric titration analysis, there has been generally utilized an inflection point method or an intersection point method using a titration curve, or a method using a differential curve derived from the titration curve. FIG. 7 is a graph showing a concept of the conventional potentiometric titration analyzing method using the intersection point method, whereas FIG. 8 is a graph showing a concept of the conventional potentiometric titration analyzing method using the differential curve.
In the above inflection point method, the titration procedure is carried out to prepare a titration curve in which the value (Y) of indicator potential is plotted on an ordinate axis and the value (X) of a volume of a titrant added to a sample is plotted on an abscissa axis to determine the abscissa axis value corresponding to an inflection point thereof as a terminal point of the titration. The inflection point method is suitably used for the analysis in which an inflection point is apparently observed. Also, in the intersection point method, as shown in FIG. 7, tangent lines (F1) and (F2) each having a gradient of 45° are drawn at the portions of a titration curve prepared in the same manner as described above which portions extend toward maximum and minimum values thereof, respectively, and then an intermediate line (G) which is spaced by an equal distance from each of the tangent lines and extend in parallel therewith is drawn, to thereby read an abscissa axis value of an intersection point (E) between the titration curve and the intermediate line (G) as a terminal point (E) of the titration. The intersection point method is a so-called drawing method and effective for the analysis of a titration curve having a pattern in which any clear inflection point is hardly recognized.
On the other hand, in the method using a differential curve, as shown in FIG. 8, an absolute value of a rate of change in potential (dY/dX) is plotted on an ordinate axis and the value (X) of a volume of a titrant added is plotted on an abscissa axis to prepare a differential curve, and the abscissa value of a peak (H) on the differential curve at which the rate of change in potential (dY/dX) becomes maximum is read out as a terminal point (h) of the titration. The method using a differential curve is suitably used for automatic titration in which automatic computation is carried out using a titration apparatus (refer to JIS K0113, 1997, Revised Edition “General Rules for methods of potentiometric, amperometric, coulometric and Karl Fischer titrations”).
Meanwhile, in the titration analysis, a titration curve is generally prepared by plotting about 10 to 20 measured values to determine the above inflection point, intersection point or peak on the curve. However, owing to the problems such as properties of the sample to be measured and poor measuring sensitivity of the apparatus, it has been difficult to obtain a smooth titration curve. Also, in the case where there is caused considerable change in indicator potential at an inflection point portion of the titration curve such as a long-continued maximum gradient portion of the curve including the inflection point, or in the case where a peak of the differential curve is unclear, there tends to occur such a problem that a terminal point of the titration curve cannot be accurately determined. When the titration curve is graphed to manually draw the tangential lines, an analysis accuracy of the titration analysis may be enhanced by skilled experts to a certain extent. However, in particular, when performing an automatic titration using a titration apparatus, it has been still difficult to estimate a smooth titration curve and therefore estimate adequate tangential lines.